SelectTrack doesn't do much if the track is already selected, but there is a small amount of overhead for any binding action. If a redundant action happens a lot, say thousands of times, this can lead to a "memory pool imbalance" which isn't correcting itself as well as it should. I don't think this would impact performance much, but you may notice a gradual increase in memory consumption over the course of an hour or so.
It's one of those things that isn't supposed to cause problems, but it's on the edge of "normal" use so I don't watch it closely. It sounds related to your auto-select-on-threshold idea which would be a good thing. I might want to make that a built-in option.
edit: actually the potential memory issue should only happen when running standalone. If you're using the plugin and getting the MIDI for bindings through the host I would not expect to see it.